1. Field of the Invention
The present invention relates to an image capturing apparatus, such as a digital still camera and a video camera, and in particular relates to an image capturing apparatus having an automatic focus adjusting function of an imaging-plane phase difference type.
2. Description of the Related Art
One type of autofocus (AF) employed for an image capturing apparatus, such as a camera, is phase-difference detection (hereinafter referred to as “phase difference AF”). The phase difference AF technique involves dividing a light beam passing through an exit pupil of the photographic lens into two light beams; these divided light beams are then received by a pair of focus detection sensors.
An amount of difference between signals output in accordance with the amount of received light by each sensor, i.e. the relative positional displacement amount in the dividing direction of the light beams (hereinafter referred to as the image displacement amount), is detected (determined) in order to obtain a defocus amount in a focusing direction of the photographic lens.
Japanese Patent Application Laid-Open No. 2001-305415 (JP 2001-305415) discloses a configuration that imparts a phase difference detecting function to an image sensor to thereby eliminate the need for a special focus detection sensor and achieve high-speed phase difference AF.
In JP 2001-305415, the photoelectric converter of a pixel in the image sensor is divided into two sections to provide a pupil-dividing function. The outputs of the two photoelectric converter sections are processed individually to detect the focus, and the sum of the outputs of the two photoelectric converter sections is used as an image signal.
Japanese Patent Application Laid-Open No. 2010-271419 discloses a configuration that reduces the effect of noise for a dark subject during focus detection performed by photoelectric converters having the pupil-dividing function. This configuration detects brightness per line in an image sensor, and performs line addition if the detected brightness is lower than a preset brightness.
Japanese Patent Application Laid-Open No. 2013-072906 discloses a technique to suppress variation in the results of the focus detection performed by photoelectric converters having the pupil-dividing function. For this purpose, a correlation calculation per line in an image sensor is performed for a plurality of lines in the image sensor and the resultant correlation amounts are added.
In the configuration described in Japanese Patent Application Laid-Open No. 2001-305415, the focus detecting function and the imaging function are both performed on the image sensor. Thus, this technique poses a challenge in that, since the exposure setting value for controlling the image sensor is determined in accordance with the imaging, this setting value is not suitable for the focus detection, which results in variability in the results of the focus detection.
The configuration described in Japanese Patent Application Laid-Open No. 2010-271419 reduces noise in image signals used for the focus detection by performing the line addition on the basis of image signals; however, it fails to take into account the reduction in resolution in a longitudinal direction of the subject due to the line addition.
The configuration described in Japanese Patent Application Laid-Open No. 2013-072906 needs to repeat the correlation calculation in order to suppress the variation in the results of the focus detection when a subject has no contrast or when noise in image signals is significant. This may result in an increase in the duration of the calculation.